1. Field of the Invention
The present invention relates to an optical scanning apparatus for used in an image forming apparatus.
2. Description of the Related Art
In image recording in an electrophotographic image forming apparatus, an image forming method using a laser has been widely used as an image forming unit that obtains high-definition image quality. In the case of the electrophotographic method, it is common to use a method in which a photosensitive drum is rotated (vertical scanning) to form a latent image, while scanning the laser (horizontal scanning) by using a polygon scanner (a rotary polygon mirror), which is one of deflectors, in an axial direction of an image carrier (for example, a photosensitive drum) having photosensitivity. In the technical field of electrophotography, there has been a requirement of forming images with higher density at higher speed. However, high density of images and image output speed are in a trade-off relationship, and balancing of both characteristics has been required.
As a method for balancing the high density of images and the image output speed, high speed rotation of a polygon scanner can be considered. However, it causes an increase in noise of the polygon scanner, an increase in power consumption, and deterioration of durability.
Therefore, as a method for preventing these problems, it is considered to use multi-beams of the light source, and there are methods as described below:
(1) a method of synthesizing optical beams from a plurality of light sources (for example, an end-face emitting semiconductor laser (LD)) (for example, see Japanese Patent Application Laid-open No. 2005-250319);
(2) a method of using an end-face emitting one-dimensional LD array in which a plurality of light emitting elements are arranged one-dimensionally; and
(3) a method of using a surface-emitting two-dimensional LD array in which a plurality of light emitting elements are arranged two-dimensionally.
In the method of synthesizing the optical beams from the plurality of end-face emitting LDs, a general-purpose semiconductor laser (LD) can be used. Therefore, this method is economical; however, it is difficult to maintain relative positions between the LDs and a coupling lens stably by a plurality of optical beams, and an interval between scanning lines formed on a surface to be scanned by the multi-beams becomes nonuniform. Further, in this method, it is difficult to have many light sources, thereby making it difficult to achieve super high density and super high speed.
Further, in the method of using the end-face emitting one-dimensional LD array, the end-face emitting one-dimensional LD array can make the interval between scanning lines uniform; however, it has problems that the power consumption of the light emitting elements increases, and if the number of beams is radically increased, a deviation amount of beams from the optical axis of an optical element in an optical system increases, thereby deteriorating optical characteristic.
On the other hand, in the method of using the surface-emitting two-dimensional LD array, a surface-emitting laser (vertical-cavity surface-emitting laser: VCSEL) is a semiconductor laser (LD) that emits beams vertically relative to a substrate, and two-dimensional integration is easy. Further, the power consumption is smaller than the end-face emitting laser by one digit, and advantageous in two-dimensionally integrating more light sources. FIG. 13 is one example of arrangement of a conventional surface-emitting laser array. In this example, 32 light emitting elements are arranged in 8×4 matrix.
Related conventional techniques are disclosed in Japanese Patent Application Laid-Open Nos. 2005-250319 and 2004-287292, as an example of a write optical system that scans by using a polygon scanner, where a distance between outermost light sources in the main scanning direction is equal to or shorter than that in the sub scanning direction. At this time, it is necessary to reduce an absolute value of lateral magnification of the optical system in the sub scanning direction, to achieve the interval between high-density scanning lines on the surface to be scanned, thereby causing insufficient quantity of light. Particularly, in the case of the surface-emitting laser device, high output is a major issue, and it causes a serious problem. Further, to reduce the absolute value of lateral magnification of the whole optical system in the sub scanning direction, an optical element having a positive power in the sub scanning direction needs to be arranged at a position closest to the surface to be scanned of the scanning optical system, so that the optical element is brought close to the surface to be scanned. Therefore, the size of the optical element closest to the surface to be scanned increases, thereby causing a cost increase. Further, a housing for mounting the optical element increases, thereby causing a problem that a mechanical size increases. Furthermore, a space between the surface to be scanned and the scanning optical system becomes narrow, a toner amount in a developing unit that can be arranged in the image forming apparatus decreases, and therefore toner replacement needs to be performed frequently. A space for arranging units for executing an electrophotographic process required for image output, such as a charger, a transfer unit, a developing unit, and a fuser decreases, and the charger, the transfer unit, the developing unit, and the fuser need to be arranged on an opposite side of the scanning optical system, thereby increasing the mechanical size.
Further, as another method for reducing the absolute value of lateral magnification of the whole optical system in the sub scanning direction, there is a method of reducing the absolute value of lateral magnification of the optical system prior to a deflector in the sub scanning direction. At this time, however, an optical element for forming a line image (for example, a cylindrical lens) close to the deflector needs to be brought close to the deflector (the polygon scanner), thereby causing a problem that the optical element for forming the line image is affected by heat generation by the polygon scanner.